This invention relates generally to nonreciprocal devices and more particularly to microwave circulators and isolators that are compatible with microwave monolithic integrated circuits.
As it is known in the art, so-called microwave monolithic integrated circuits include active and passive devices which are formed using semiconductor integrated circuit techniques to provide various types of microwave circuits. One such particular application of this technology is in so-called transmit/receive modules (transceiver modules) for use in phased array antennas. In these so-called transceiver modules, active devices such as field effect transistors are combined with passive devices such as capacitors, resistors, inductive elements, and the like to form various microwave functions such as amplifiers and switches. In certain applications, it would be desirable to provide one or more circulators to steer electromagnetic energy through such transceiver modules.
Although non-reciprocal microwave components such as circulators and isolators have been known for many years, no convenient approach is available for combining the circulator function on a common substrate which supports the integrated circuits. Generally, these nonreciprocal components are fabricated separately from the semiconductor components of the transceiver on different substrates.
The nonreciprocal components such as circulators are usually fabricated either on a ferrite substrate or on a dielectric substrate that has a ferrite insert. Both methods of fabrication have the disadvantage of requiring relatively long connecting transmission lines between the integrated circuits and the circulator with associated transmission loss. This hybrid-type approach is also relatively labor intensive and, therefore, costly.
The circulators which are known for use with microwave integrated circuits are usually designed for coupling by means of microstrip transmission lines. An example of a circulator which is well known in the art is shown in FIG. 1. The circulator 10 in FIG. 1 uses a dielectric substrate 12 having a ground plane conductor 14 and a ferrite insert 18. In this design the strip conductors of the microstrip lines 16a-16c are connected to a metal disc 19 that has approximately the same diameter as the ferrite disc 18. Critical design parameters for this type of circulator are the radius (R) of the metal disc 19 and the coupling angle (.psi.), which is related to half the angle subtended by each microstrip at the perimeter of the central disc 19. A circulator design similar to one shown in FIG. 1 has the capacity for a very large bandwidth.
Another type of circulator element 25, which is also well known in the art, is shown in FIG. 2. This circulator element 25 is also based on microstrip transmission medium and also uses a ferrite 23 having an "interwoven" coupling structure 25 with port conductors 25a-25c fabricated on the surface of the ferrite disc 23. Three mask levels are required to fabricate this coupling structure by photolithography (two levels for metalization and one level for a dielectric layer which separates the two layers of metalization). The port conductors on the disc are bonded to corresponding strip conductors 26a-26c formed on the substrate.
For a miniature type of circulator, one could insert circulator element 25 into a dielectric substrate 22 having a ground plane conductor 24, as shown. A hole 27 must be provided in such a dielectric 22.
The advantage of a circulator design such as illustrated in FIG. 2 is that it would achieve significant size reduction compared to the design shown in FIG. 1. The circulator illustrated in FIG. 2 includes capacitor pads 29a-29c disposed on the surface of the dielectric substrate at the terminus of the strip conductors 26a-26c, and a capacitor 28 provided by a metal layer 28a disposed on the bottom surface of ferrite 23, dielectric 28b, disposed thereover and a second metal layer 28c which acts as a ground plane conductor disposed on the dielectric 28b. The size of the capacitor pads 29a-29c depends upon the frequency of operation with lower frequencies requiring relatively large capacitor pads. The capacitor is disposed on the bottom of the ferrite disc 23 to capacitively couple the node metalization 28a to the ground plane conductor. The capacitances of the capacitors formed over the ferrite 28 and strip conductors 26a-26c are adjusted to achieve optimum performance. Theoretical calculations concerning the design of the circulator element 25 (FIG. 2) and actual performance data for the element 25 have been reported by R. Knerr, IEEE Transactions MTT-18, pp. 1100-1108, Dec. 1970.
Therefore, although circulators are known to be fabricated on dielectric substrates, the problems with the design illustrated in FIGS. 1 and 2 are that each circulator requires a hole to be drilled into the dielectric substrate to receive the ferrite disc. This requirement makes this general approach to integrating circulators with semiconductor circuits comparatively costly. The hole requirement also presents a very difficult fabrication problem particularly when the circulators are selected to be fabricated on brittle dielectric substrates such as gallium arsenide, the generally preferred material for monolithic microwave integrated circuits. Since gallium arsenide is a relatively brittle material, fabrication of the hole in gallium arsenide is not a trivial process step.
Further, each of these approaches requires that the coupling structure, as well as capacitors, be fabricated on or under the ferrite disc. This is also a difficult processing step.
Fabrication difficulties can be avoided with these techniques by simply using a substrate without a hole and placing the ferrite disc with the coupling structure on top of the junction and thus, otherwise retain a structure as shown in FIGS. 1 or 2. It has been shown, however, that this technique does not lead to circulators with useful performance, since the electromagnetic field does not penetrate sufficiently into the ferrite but remains concentrated in the dielectric substrate.